John, I guess, in the generally scheme of things, the insulating firebrick would probably guarantee you full duration broiling better than the kiln post, but I kind of the like the kiln post because it's an existing item that doesn't require any modification. If, say, the broiler cuts out at 550, you can pre-heat the oven to 545 (most likely enough for a Neapolitan bake on the bottom), then crank the broiler, and, by the time the heat makes it through the post to raise the probe 5 degrees, your pizza will have been baked.
The other appealing aspect about the kiln post is that eventually it will heat up and the element will shut off- for both the baking element and the broiling element. The insulating brick will block out the heat so effectively that you're going to have to watch the pre-heat carefully so that the oven doesn't burn up. With the insulating brick in place, neither element will ever cut off.
I don't know, maybe the foil will turn out to be viable. It's been hit and miss for Tyler, but perhaps there's a way to craft it a bit better. The nice potential thing about foil is that, unlike insulating brick, it will let heat through, and, unlike cordierite, if it heats up, it won't stay hot for long. All you really need is a material that's just insulating enough to keep the broiler on for 90 seconds when the oven is at it's peak temp- and no more.